A. Field of the Invention
B. Description of the Related Art
There are many commercial applications for thin films and, in particular, multilayer films. One particularly promising application is the use of these films in fiber-optic networks. Multilayered films are used in Dense Wavelength Division Multiplexers/Demultiplexers (DWDM) systems which enable information to be delivered inside the fiber optic cables at multiple wavelengths.
The ability to transmit data via fiber optic cables has become of increasing importance in this technological age. At the present time, the installation of a worldwide fiber-optic network is in progress that will be capable of handling levels of data transmission inconceivable only several years ago. As a result of this network, the Internet is less than half a decade away from being a more useful tool than the computers which navigate it. As the biggest technological revolution in the history of modem civilization progresses, advanced high performance coatings have emerged as the enabling technology. The ability to control transmission and reflection of selected wavelengths of light has enabled existing fiber to accommodate the increase in bandwidth which will be required over the next 3-5 years.
Dense Wavelength Division Multiplexers,/Demultiplexer (DWDM) systems enable information to be delivered inside fiber-optic cables at multiples wavelengths. The increase in the bandwidth is limited only by the number of wavelengths which can be superimposed on the fiber. Current state-of-the-art DWDMs can multiplex/demultiplex approximately 130+channels. Ultimately more than 1000 channels will be possible. During transmission, information is packaged within phase modulated carriers at specific wavelengths and superimposed (multiplexing) on the fiber. During reception, the carriers must be separated (demultiplexing). Optical component technology such as DWDMs are critical to achieve bandwidth necessary for future interactive services such as xe2x80x9cvideo on demandxe2x80x9d, and have prompted multi-billion dollar strategic acquisitions such as OCLI, NetOptix, and XROS.
The most widely used technology for multiplexing and demultiplexing in DWDM systems is thin film-based. Multilayered thin dielectric coatings are comprised of 150-200 layers with individual optical layer thickness equal to multiples of xc2xc of the wavelength to be transmitted (known as dielectric interference filters). A collection of such filters coupled together, each differing slightly in design to allow light transmission of different wavelengths, and xe2x80x9cconnectedxe2x80x9d to a fiber-optic cable enables the multiplexing (superposition) and demultiplexing (separation) of multiple wavelengths of laser light containing digital information.
Current thin film multiplexers and demultiplexers can handle up to 40 different wavelengths but several manufacturers have announced 80 channel versions in year 2000. With current state-of-the-art deposition processes used for DWDM, 80 channel multiplexers will approach the limit of the technology. Theoretical thin film filter designs exist with Full Width at Half Maximum (FWHM) of less than 0.1 nm. Such a filter would enable multiplexers capable of handling more than 1000 channels.
Surface roughness reduction and interface smoothing by ion bombardment has been examined extensively for multilayered films designed for x-ray reflectors. In that collection of work it was observed that, by xe2x80x9cion polishingxe2x80x9d the film surfaces using Ar+ or O+ ions accelerated from an ion source, surface roughness (Ra was reduced by a factor of 2 as is shown in FIG. 1. It was also observed that deposition of a thin amorphous carbon (C) layer at each interface, between layers of multi layered reflectors, was successful at reducing interface roughness.
It is well known that a very hard low surface roughness amorphous carbon coating be deposited with various ion processes including ion beam deposition (IBD) and plasma enhanced chemical vapor deposition (PECVD). These coatings are used primarily for anti-abrasion and as antireflective surfaces on germanium substrates for infrared transmissive windows.
Diamond-like carbon (DLC), and other forms of amorphous carbon, can be stripped from substrates by exposing the surface to an energetic ( greater than 50 V) oxygen plasma. The energetic oxygen ions react chemically with the carbon surface to form carbon monoxide (CO). The vapor pressure of CO is high enough, at the vacuum level at which this process is performed (xcx9c10xe2x88x922 torr), that the CO molecules xe2x80x9cevaporatexe2x80x9d from the surface. The freshly exposed surface carbon then reacts with the plasma and the process continues until the oxygen plasma is extinguished or no amorphous carbon remains.
The present invention contemplates a new and improved process for reducing the surface roughness of thin films which is simple in design, effective in use, and overcomes the foregoing difficulties and others while providing better and more advantageous overall results.
In accordance with the present invention, a new and improved process is provided which reduces the surface roughness of thin films at the atomic level.
In accordance with another aspect of the present invention, a surface planarization process for planarizing vacuum-grown thin films includes the steps of depositing a thin layer of silicon dioxide onto a near-atomically flat fused silica substrate, depositing approximately a one-nanometer thick first layer of amorphous carbon onto the silicon dioxide, directing a well-focused oxygen ion beam onto the carbon coated silicon dioxide at near grazing incidence, rastering the ion beam in a sweeping fashion to allow interaction with only the carbon which protrudes above average surface height, the rastering being continued until a top layer of carbon is reduced to the level of the highest peaks in the thin film, depositing a thin layer of a titanium dioxide onto the carbon coated silicon dioxide, depositing approximately a one-nanometer thick second layer of amorphous carbon onto the titanium dioxide, directing a well-focused oxygen ion beam onto the carbon coated titanium dioxide at near-grazing incidence, rastering the ion beam in a sweeping fashion to allow interaction with only the carbon which protrudes above average surface height, the rastering being continued until a top layer of carbon is reduced to the level of the highest peaks of the thin film, and repeating the process as necessary.
In accordance with still another aspect of the present invention a process for planarizing thin film surfaces includes the steps of depositing a material onto a near-atomically flat substrate, depositing carbon onto the material, directing an ion beam onto the carbon coated material, rastering the ion beam until the carbon is reduced to approximately the level of the highest peaks of the material, depositing a second material onto the carbon coated material, depositing carbon onto the second material, directing an ion beam onto the carbon coated second material, rastering the ion beam until the carbon is reduced to approximately the level of the highest peaks of the second material, and the process is repeated as necessary.
In accordance with yet another aspect of the present invention, the process includes depositing a low index material onto a near-atomically flat fused silica substrate, depositing a high index material onto a carbon coated low index material, depositing approximately a one-nanometer thick first layer of amorphous carbon onto the low index material, and depositing approximately a one-nanometer thick second layer of amorphous carbon onto the high index material.
In accordance with another aspect of the present invention, the process includes depositing a thin layer of silicon dioxide onto a near-atomically flat fused silica substrate, depositing a thin layer of a material, chosen from the group comprising: titanium dioxide and tantalum pentoxide, onto a near-atomically flat fused silica substrate, rastering the ion beam in a sweeping fashion to allow interaction with only the carbon which protrudes above average surface height, the rastering being continued until a top layer of carbon is reduced to the level of the highest peaks in the thin film, directing a well-focused oxygen ion beam onto the carbon coated silicon dioxide at near grazing incidence, and directing a well-focused oxygen ion beam onto the carbon coated titanium dioxide at near grazing incidence.
In accordance with another aspect of the present invention, the process includes depositing a thin layer of silicon dioxide onto a near-atomically flat fused silica substrate, the silicon dioxide being deposited via a process chosen from the group comprising: ion beam deposition, ion beam sputter deposition, molecular beam epitaxy, and atomic layer epitaxy.
In accordance with another aspect of the present invention, a thin film filter design includes a near-atomically flat substrate, at least one low index layer, at least one high index layer, and at least one carbon layer on each of the at least one index layers, the at least one carbon layer being reduced by an ion beam.
In accordance with still another aspect of the present invention, the film is a dielectric film with a thickness of approximately 150 nanometers, the dielectric film has a surface roughness of approximately 0.05 nanometers, the substrate is fused silica, the at least one low index layer is silicon dioxide and the at least one high index layer is titanium dioxide, the ion beam is a well focused oxygen ion beam, and the at least one low index layer and the at least one high index layer are layered alternately.
To accomplish these objectives, an oxygen ion process, Chemical Reactive-Ion Surface Planarization (CRISP), has been developed which enables planarization of thin film surfaces at the atomic level. Narrow/broad band filters produced with vacuum deposited multilayered thin films are designed to selectively reflect/transmit light at specific wavelengths. The optical performance is limited by the ability to control the individual layer thickness, the xe2x80x9croughnessxe2x80x9d of the individual layer surfaces and the stoichiometry of the layers. The process described here will enable reduction of surface roughness at the interfaces of multilayered thin films to produce atomically smooth surfaces. The application of this process will result in the production of notch filters of less than 0.3 nm full width at half maximum (FWHM) centered at the desired wavelength. This will enable optical filters designed for telecommunication components such as next generation dense wavelength division multiplexer (DWDM) systems with significant performance improvement beyond the state-of-the-art.
Still other benefits and advantages of the invention will become apparent to those skilled in the art upon a reading and understanding of the following detailed specification.